Snowbound

According to the holiday carol, someone is dreaming of a white Christmas. This season, Shrek, Fiona, the kids, Donkey, and Puss In Boots are dreaming of one too, as they get ready to experience an unforgettable Christmas. In the made-for-television special Shrek the Halls, airing this month on ABC, Shrek’s murky swampland is transformed into a snowy, glistening winter wonderland, thanks to the digital artists at DreamWorks, who worked their own holiday magic to accomplish this task.

The half-hour show is a continuation of this past May’s Shrek the Third, as the ogre is ready to finally sit back, relax, and enjoy his happily ever after with his new family. But can he do that during the most joyous—and busiest—of holidays? His plans for a quiet, cozy family celebration are interrupted when the rest of the gang have their own ideas about what Christmas is all about.

The show reunites the DreamWorks Shrek gang, as well. Shrek the Third’s CG supervisor, Philippe Denis, who worked on every Shrek project thus far, including the Shrek 4D ride film at Universal Studios, returns as visual effects supervisor on Shrek the Halls. In fact, nearly all of Shrek the Hall’s 200-member crew moved over to the holiday show after completing their work on the third film. Preproduction of the TV special began late last year when the movie was in full production.

Most of the program’s main cast appeared in Shrek the Third, enabling the team to repurpose the character models and rigs. The characters had been reworked for that film from the ground up using PDI/DreamWorks’ proprietary software, with new controls for animation and an increased range of motion (see “Merry Tales,” April 2007). Most of the Christmas story takes place in Shrek’s swamp, allowing the artists to reuse that environment, as well. The artists also repurposed Olde Town and a number of other assets, including trees for the pine forest. They also “regifted” the Christmas tree in the short film “The Madagascar Penguins in a Christmas Caper,” growing the evergreen to ogre proportions.

Repurposing assets saved the artists a good deal of time, but there was just one problem. In the previous Shrek movies, the weather outside is more delightful. For Shrek the Halls, it is more frightful; it is, after all, wintertime. “We had to change the overall look because there is snow everywhere,” says Denis. “It wasn’t as simple as using the old environment and just adding snow.”

Let It SnowWhen the team started development on the snow, it had to overcome a number of challenges. The first one was purely aesthetic: how to make the snow look like snow. The second was more technical: how to cover the Shrek environment, which comprises many different elements—big surfaces, specific models, and scattered objects—with snow. The third entailed consistency: how to seamlessly integrate these different types of snow.

Rather than applying a single solution, the team used a range of methods to achieve the various types of snow: blanket snow, falling snow, blowing snow, each with its own look. To do so, various departments—modeling, surfacing and effects, and lighting—had to work closely together.

Ground Cover For the big surface snow, the artists first turned to modeling, which created a snow plane atop large objects: the ground layer, Shrek’s house, and the base and large branches of the trees, for instance.

For the smaller objects scattered within the environment, the team devised a procedural method using a proprietary system that added finer particles to the items and brought out their details. Using this proprietary Snow Cover System, the artists started with default attributes, including particle size, and then dialed them up or down for an even covering of snow.

“There are so many assets in the swamp, lots of cattails, grass, bushes, trees with thousands of leaves. To cover it in snow using the modeling method would have taken years,” says David Lipton, head of effects. “We tried to come up with ways to procedurally handle it and still get little details—for instance, snow blobbing together to connect two blades of grass with a single surface—as opposed to snow just resting on top of objects.”

The system also determined snow placement—basically layers on top of the original geometry—while giving the production designers control over the aesthetic. Lipton explains the process used to determine which objects would be covered in snow: “We would shine a light directly from above straight down onto the geometry below that we wanted to cover in snow. The light produced a shadow; anything in shadow would be dark and not get snow.” For example, if a light were shining on a table, the top would be lit while underneath it would be in shadow and not have the snow.

In many scenes, the crew had to balance the blue light of night with the warm light shining through the windows of Shrek’s house.

“We converted the entire environment into a particle pass wherein every object was covered by particles, and those particles would have the lighting baked in,” Lipton explains. “We then took the lighting information and removed any particle whose color was less than a certain value, like .1 or 10 percent gray. It then removed all the information where there shouldn’t be snow.”

Referring once again to his earlier example, Lipton notes that the particles on top of the table would be white, because they are lit, but the particles underneath the table would be black.

Once the shape of the object took form, the particles were turned into geometry, and then shaded, textured, and lit.

“We looked at a lot of visual reference and defined what was making the snow look so specific,” says Denis. “In particular, we were trying to achieve fine surface detail (displacement as well as diffusion), translucency, and glitter in close-up shots by making the light diffuse on the snow surface.” A glitter shader added the sparse distribution of sparkle over the soft snow, adding some textural detail.

Later, the surfacing group painted masks and added noise elements to integrate the areas with the surface-modeled snow and those without.

Snow WhiteAfter examining a plethora of pictures and other visual reference, the artists noticed that snow seemed to look different, especially when examining it from a distance. “We realized the snow reacted differently with the lighting depending on the angle formed between the surface and the direction of the light,” says Denis.

Thus, the effects artists used the angle between the light direction and the normal of the surface to play with the color and the saturation. In other words, if the normal of the surface was parallel with the direction of the light, they used one lighting color. If it was diverging, they often used a darker and more saturated version of the same hue. This process emphasized the shape of the element, or the terrain, that was being lit, which is often a challenge when there is a white surface like snow.

The white surface also challenged the group to maintain the volume and space in both night and day shots. “It’s easy to say, ‘bring up the brightness in the night shots,’ but you still have to keep it nighttime,” says Denis.

As Lipton points out, the show contains a number of night scenes, with lots of blues and oranges seen around the firelight. “For effects, a big challenge was the blue light of night and warm light from the interior of Shrek’s house. You have blue and orange, and when you light the particles, the result is purplish,” explains Lipton. “In many instances, we used the background render and pulled keys where there was orange and blue, so when we lit the particles, the color of the background pushed through the color of the particles in the comp. It’s not like lighting geometry where you have a continuous smooth surface.”

Falling SnowIn addition, Shrek the Halls called for falling snow, achieved using an Autodesk Maya particle simulation created by effects artist Steve Wood. In the beginning, the snow falls lightly, eventually turning into heavily falling snow. For this, the artists added more noise and turbulence to the simulation. They also played with the atmosphere, using 3D volume rendering mixed with a 2D technique, like a pixel shader, as well as particle rendering, during the peak of the snowfall.

Mark Manfrey, an effects artist, then examined each shot to make sure the density and brightness was consistent, since unique simulations were used throughout.

As the snow begins to fall, flakes (particles) land on the characters for just a second before gently melting, done by applying a limited lifespan on the particles. In other shots, the snow mixes with Donkey’s fur, an effect that was painted on with particles to give the effects artists more control over where the snow landed and gathered.

Just about every object outdoors is covered in snow. The big surface snow was created in modeling, while a procedural Snow Cover System was used to “dust” smaller objects.

A number of shots also have the characters interacting with the snow in both subtle and not-so subtle ways. “For the blanket and ground snow, we had a formulated system, but for the interactions, the artists were free to come up with their own technique,” says Lipton, who tweaked the splash emitter he developed for the sand in Madagascar to generate the snow “kick-up.”

“Some artists did cool things like create tiny snowballs that were rigid-body simulations, but the rigid-body object would leave particles behind, and they would be used as a displacement map. So you get little snowballs that roll down a hill, but they leave a little trail in the path,” explains Lipton.

BrrrrrThe same technique used to create the atmosphere in the snowstorm was also used to produce a “chilling” effect. “When we started to light the scenes, we realized we were missing atmosphere. We didn’t feel as though it was cold enough outside,” says Denis. “So we used volume rendering to produce some fog.”

To accentuate the cold, the artists added a cold-breath effect—the foggy haze that surrounds a character’s mouth when the warmth from the breath comes in sudden contact with the freezing air. Although subtle, the effect, produced with particles and a little dose of light, adds atmosphere. “You can tell it is very cold,” Denis says about these outdoor scenes.

Initially, Lipton did not think there was enough time to integrate this effect due to the sheer number of shots involved. But upon Denis’s insistence, Lipton challenged a senior-level artist to come up with an easy but effective solution. He did: The effect would be generated automatically as a simulation that was attached to the lip-sync animation. The script would analyze the character’s top and bottom lips in Maya, and track how far apart the lips were. It would then examine the previous frame. If the lips were farther apart on the current frame, the assumption was that the mouth was opening, not closing, and, thus, would emit cold-breath particles.

“The artists could go to a shot, choose the characters, run two scripts, submit a render, and get something back in less than an hour,” explains Lipton. “They could review it, tweak it, and have the shot done the next day.” Lipton estimates that this system, which was integrated into the pipeline within just a few days, got the job done 95 percent of the time.

Shrek TechShrek the Third broke new technical ground on a number of fronts, as did the previous films. But for Shrek the Halls, it was more a matter of mixing techniques to accomplish what was necessary.

“It was a fast project, and we had to say, ‘Here are the tools we have. How can we put them together to make things work and still be creative?’” says Denis. “It was a fun project. Everyone came together from various departments to work together.”

And that’s what holidays, and family, is all about.

Karen Moltenbrey is the chief editor for Computer Graphics World.

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